This invention relates generally to measurement of gas cluster size, and, more particularly to measurement of mean gas cluster ion size.
The use of a gas cluster ion beam (GCIB) for etching, cleaning, and smoothing of the surfaces of various materials is known in the art (See for example, U.S. Pat. No. 5,814,194, Deguchi, et al., xe2x80x9cSubstrate Surface Treatment Methodxe2x80x9d, 1998). For purposes of this discussion, gas clusters are nano-sized aggregates of materials that are gaseous under conditions of standard temperature and pressure. Such clusters typically consist of aggregates of from a few to several thousand atoms or molecules loosely bound to form the cluster. These clusters can be ionized by electron bombardment or other means, permitting them to be formed into directed beams of known and controllable energy. The larger sized clusters are the most useful because of their ability to carry substantial energy per cluster ion, while yet having only modest energy per atom or molecule. The clusters disintegrate on impact, with each individual atom or molecule carrying only a small fraction of the total cluster energy. Consequently the impact effects of large clusters are substantial, but are limited to a very shallow surface region. This makes ionized clusters effective for a variety of surface modification processes, without the tendency to produce deeper subsurface damage characteristic of monomer ion beam processing.
Means for creation of and acceleration of such GCIB""s are described in the Deguchi reference previously cited. Presently available ionized cluster sources produce cluster ions having a wide distribution of sizes, N (where N=the number of molecules in each clusterxe2x80x94in the case of monatomic gases, an atom of the monatomic gas will be referred to as a molecule, or cluster of size N=1, and an ion of such a monatomic gas will be referred to as a molecular ion, or an ionized cluster of size N=1, or a cluster ion of size N=1, throughout the following discussion). The cluster formation process has been shown by N. Kofuji, et al. (in xe2x80x9cDevelopment of gas cluster source and its characteristicsxe2x80x9d, Proc. 14th Symp. on Ion Sources and Ion-Assisted Technology, Tokyo (1991) p. 15) to produce few small size clusters (values of N from 2 to about 10), but molecular ions (N=1) are produced in abundance as are larger clusters (N greater than a few tens, up to several thousands.) It is known (U.S. Pat. No. 5,459,326, Yamada, xe2x80x9cMethod for Surface Treatment with Extra-Low-Speed Ion Beamxe2x80x9d, 1995) that atoms in a cluster are not individually energetic enough (on the order of a few electron volts) to significantly penetrate a surface to cause the residual sub-surface damage typically associated with the other types of ion beam processing in which individual monomer atoms may have energies on the order of thousands of electron volts. Nevertheless, the clusters themselves can be made sufficiently energetic (some thousands of electron volts), to effectively etch, smooth or clean surfaces as shown by Yamada and Matsuo (in xe2x80x9cCluster ion beam processingxe2x80x9d, Matl. Science in Semiconductor Processing I, (1998) pp 27-41).
To a first order approximation, the surface modification effects of an energetic cluster are dependent on the energy of the cluster. However, second order effects are dependent on the velocity of the cluster, which is dependent on both the energy of the cluster and it""s mass (and hence the cluster size, N.) In order to maximize the utility of a GCIB for surface processing, it is useful to know and control both the energy of the clusters and the mean cluster size or the cluster size distribution. In certain applications gas cluster ion beams are used for deposition or growth of surface films. When so used, it is important to know the mass flow to the workpiece. The quantity of ions is readily determined by measuring the ion current that reaches the workpiece. Since it can be arranged so that the ionized clusters predominately carry a single electrical charge, it can be accurately assumed that each charge corresponds to a single ionized cluster or molecular ion, but unless the mean cluster size or cluster size distribution is known, the total mass flow to the target is not known. It is possible, by controlling the source conditions to influence both the ratio of cluster ions to molecular ions and the cluster size distribution (and thus the mean cluster size). However, unless a means is available to measure and monitor the mean cluster size or cluster size distribution, adjustment and control of the source to produce desired cluster sizes is difficult. For these and other reasons it is useful to have a measurement means that can provide information about cluster size in a gas cluster ion beam. A simple, compact, and inexpensive means of measuring the mean cluster mass in beam is desirable for diagnosing operation of a cluster source and ionizer.
In addition to cluster ions, a GCIB is likely to have a significant number of unionized clusters and molecules traveling with the ionized beam. Although a minor fraction of such unionized particles may include ions that have become neutralized through collisions, the majority consists of clusters and molecules that did not ionize while transiting the ionizer. Unionized clusters and molecules cannot be accelerated like ions, and consequently, have only thermal energy. These low energy unionized clusters and molecules do not participate substantially in processing a workpiece, but are indicative of the ionizer efficiency. For this reason, it is useful to have a measure of their magnitude.
Because molecular ions, as well as cluster ions, are produced by presently available cluster ion beam sources, molecular ions (cluster ions having N=1) are accelerated and transported to the workpiece being processed along with the cluster ions. Molecular ions, having high energy with low mass, have high velocities, which allow them to penetrate the surface and produce deep damage that is likely to be detrimental to the process. Such sub-surface ion damage is well established and well known from the more traditional monomer ion beam processing art and can produce a variety of damage and in implantation beneath the surface.
It has become known in the ionized cluster beam art that many GCIB processes benefit from incorporating means within GCIB processing equipment for eliminating molecular ions from the ionized cluster beams. Electrostatic (See for example U.S. Pat. No. 4,737,637, Knauer, xe2x80x9cMass Separator for Ionized Cluster Beamxe2x80x9d, 1988) and electromagnetic (For example, Japanese laid open application (kokai) 03-245523, Aoyanagi, et al., xe2x80x9cManufacture of Quantum Well Structurexe2x80x9d, 1991, cited as prior art in U.S. Pat. No. 5,185,287) mass analyzers have been employed to remove light ions from the beam of heavier clusters. Electrostatic and electromagnetic mass analyzers have also been employed to select ionized clusters having a narrow range of ion masses from a beam containing a wider distribution of masses (See previously cited U.S. Pat. No. 4,737,637 and also Japanese laid open application (kokai) 62-112777, Aoki, xe2x80x9cApparatus for Forming Thin Filmxe2x80x9d, 1987).
Presently practical GCIB sources produce a broad distribution of ionized cluster sizes, but have limited cluster ion currents available. Therefore it is not practical to perform GCIB processing by selecting a single cluster size or a narrow range of cluster sizesxe2x80x94the available fluence of such a beam is too low for productive processing. It is preferred to reduce or eliminate the molecular ions from the beam and use the remaining heavier ions for processing.
It is therefore an object of this invention to provide a way of measuring the mean cluster ion size in GCIBs.
It is also an object of this invention to provide a way of measuring the mean cluster size present in a partially unionized GCIB.
Another object of this invention is to enable determining the relative quantities of ionized and unionized material in a GCIB.
One more object of this invention is to provide a means of measuring the molecular mass flow in a GCIB, both ionized and unionized.
It is a still further object of this invention to provide a GCIB processing system wherein mean cluster size measurement facilitates the operation, adjustment, and control of the processing system.
The objects set forth above as well as further and other objects and advantages of the present invention are achieved by the embodiments of the invention described hereinbelow.
This invention involves a detector and its use in measuring mean size of gas cluster ions in a beam. The detector includes an electron suppressed Faraday cup with a high conductance path to a neutral gas pressure detector (which can comprise a commercial compact ion pressure gauge) and a high conductance to the detector exit. The apparatus is both used to acquire ion current, which is a measure of the ion beam flux, and to acquire mass flux, through a pressure measurement. Since the pressure measurement responds to the completely dissociated clusters in real time, when combined with information about instantaneous ion current, the mean cluster ion size ({overscore (N)}i) can be calculated.
For a better understanding of the present invention, together with other and further objects thereof, reference is made to the accompanying drawings and detailed description.